Effects of MCI-154 and caffeine on Ca(++)-regulated interactions between troponin subunits from bovine heart

We studied the effects of MCI-154 (6-[4-(4-pyridyl)aminophenyl]-4,5- dihydro-3(2H)-pydridazinone hydrochloride) and caffeine on the regulation of Ca++ in the interaction between troponin subunits from bovine hearts by using fluorescence spectroscopy. The titration of MCI-154 to bovine cardiac troponin C (TnC) resulted in a concentration-dependent change in intrinsic fluorescence, indicating that MCI-154 may induce conformational changes and perturb the microenvironments of the intrinsic fluorophores. MCI-154-induced changes in the rotational correlation times obtained from fluorescently modified TnC and its complex with cardiac troponin I (TnI) indicate that the effect of MCI-154 may be highly correlated with a gross alteration in the overall molecular shape of TnC. A right shift in the-fluorescence curve with a significant decrease in maximal fluorescence intensity was observed in the presence of MCI-154 in isolated TnC, indicating a decrease in the affinity of TnC for Ca++. However, in the reconstituted binary (TnC.TnI) and ternary (TnC.TnI.TnT) complexes the pCa-curve was shifted to the left under similar conditions. These data indicate that the complexation of TnC with TnI and TnT resulted in an increase in the affinity of TnC for Ca++ in the presence of MCI-154. In the studies of caffeine effect, the results showed that the effect of caffeine on isolated TnC was similar to that of MCI-154. Caffeine (0.5 microM) shifted the pCa-fluorescence curve to the right by 0.4 U in the binary complex and to the left by 0.2 U in the ternary complex at pCa50% when compared to the curve in the absence of caffeine.(ABSTRACT TRUNCATED AT 250 WORDS)